Typically, semiconductor laser diodes (Fabry-Perot laser diodes) are stable at a particular wavelength over a very narrow range of temperatures, i.e., approximately a few degrees. The reason for this instability is that adjacent longitudinal modes all have approximately the same net threshold gain (lasers operate at the wavelength with the smallest net threshold gain) In order to circumvent this problem, laser diodes have been developed which favor longitudinal modes of specific wavelengths. These types of laser diodes are generically called, frequency-stabilized laser (FSL) diodes. The two main types of FSL diodes are distributed-feedback (DFB) laser diodes (K. Aiki et al.,`GaAs-AlGaAs distributed-feedback diode lasers with separate optical and carrier confinement,` Appl. Phys. Lett., vol. 27, pp. 145-146, 1975) and distributed Bragg reflector (DBR) laser diodes (H. Namazaki, M. K. Shams, and S. Wang, `Large-optical-cavity GaAs- (GaAl)As injection laser with low-loss distributed Bragg reflectors,` Appl. Phys. Lett., vol. 31, pp. 122-124, 1977). The basic operating principle of these laser diodes is that for a very narrow range of wavelengths (called DFB modes) adjacent to the Bragg wavelength, .lambda..sub.B optical feedback is provided not only by the end-facet mirrors (as for the Fabry-Perot modes), but also by a periodic variation of the index of refraction along the longitudinal length of the laser. Accordingly, wavelengths near .lambda..sub.B have smaller losses than the out-of-phase Fabry-Perot modes, resulting in DFB laser diodes exhibiting a wavelength stability on the order of 50.degree. C. Frequency-stabilized laser diodes are particularly useful for applications such as fiber-optic communications, where wavelength stability reduces unwanted chirping effects and enables the lasers to operate at higher bit rates, and optical recording, where wavelength stability reduces the effect of optical feedback from the recording media.
For systems applications, the laser diode's output is typically coupled either to optical fibers or recording media. In order to maximize the coupling, it is desirable that the beam be circular. The shape of the output beam can be described in terms of the far-field beam divergence ratio .rho.=.theta..sub.t /.theta..sub.l, where .theta..sub.t and .theta..sub.l are the far-field divergences in the planes perpendicular and parallel to the laser junction, respectively. Typical DFB laser diodes have beam divergence ratios on the order of 4.0. Recently, there has been some activity devoted to designing non-frequency stabilized laser diodes with more circular output beams. Both Yuri et al. (M. Yuri, A. Noma, I. Ohta, and M. Kazumura, `Reduction of beam divergence angles perpendicular to the junction planes by modulating the refractive index profile in AlGaAs laser diodes`, presented at the Fall 1991 meeting of the Japanese Society of Applied Physics) and Cockerill et al. (T. Cockerill, J. Honig, T. DeTemple, and J. Coleman, `Depressed index cladding graded barrier separate confinement single quantum well heterostructure laser,`Appl. Phys. Lett., vol. 59,2694, 1991) have introduced depressed-index cladding layers into their Fabry-Perot laser diodes to significantly lower .theta..sub.t and therefore, .rho.. A schematic of an AlGaAs-based laser diode containing these layers in shown in FIG. 1. In the figure is indicated the Al content of the various layers, where 10 refers to the n.sup.+ -GaAs substrate. On the surface of 10 is formed the lower cladding layer 12. Upon 12 is deposited the lower depressed-index cladding layer 14. The index of refraction of this layer is smaller than that of the surrounding layers since the index of refraction of AlGaAs materials is smallest for pure AlAs. On the surface of 14 is formed the lower spacer layer 16, followed by the active layer 18 and the upper spacer layer 20. Upon 20 is formed the upper depressed-index cladding layer 22 followed by the upper cladding layer 24. Lastly, upon the surface of 24 is formed the capping layer 26. Since light avoids low-index regions, the physical effect of the inclusion of the depressed-index cladding layers is to push the transverse-confined waveguide mode both toward the middle and ends of the structure. With greater light intensity present in the upper and lower cladding layers, .theta..sub.t decreases as desired. .GAMMA. remains approximately stationary since light is also pushed towards the middle (active layer) of the structure. More specifically, Cockerill et al. determined that for a broad-area graded-index separate confinement heterostructure device, .theta..sub.t was 27.degree. and 59.degree. for structures with and without the inclusion of the depressed-index cladding layers, respectively.
From the above discussion, it is highly desirable to combine frequency stabilization and small transverse beam divergence. Based on manufacturability considerations, it is also desirable to use a ridge waveguide design for obtaining lateral confinement. However, there are problems inherent in combining DFB frequency stabilization with conventional depressed-index cladding structures. In order to obtain the desired redistribution of the modal-field, the depressed-index cladding layers are typically placed adjacent to the active layer. As a result the modal field surrounding the active layer is significantly reduced compared to its value in the absence of depressed-index layers. Consequently, the interaction of the field with both the DFB grating and the rib is weakened, resulting in smaller coupling coefficients and poor lateral confinement. Also, in general, structures employing depressed-index cladding layers can develop instabilities in the modal-field as a function of rib-etch depth.